A recent 'Window on Science' trip to the US Air Force
Phillips Laboratory in California led Dr Peter Beaumont deep into the Mojave
desert and into a new area of exciting research. Whilst there, he showed a video
of the microscopical processes of crack growth in an epoxy-based material toughened
by micron-sized rubber precipitates. His American colleague immediately realised
that what he was being shown was a reversal of the types of processes that are
thought to occur in solid rocket propellant fuels.

These fuels consist of rigid brittle particles in an extensible
elastomeric matrix rather than the other way round. Crack propagation in solid
propellants is of particular concern because it can result in the fuel becoming
inherently unstable and subsequently capable of premature explosion on the ground
or early in the flight mission. This is because vibrations or thermal fluctuations
during their storage, on firing or in flight, could bring about the nucleation
of small cracks in the propellant that might lead to its instantaneous ignition
rather than a controlled burn.

The Big Bang theory

Application of theories of micromechanisms of crack formation
could be useful in determining how micro-cracks start in the propellant and
may therefore indicate ways of optimising its microstructure to prevent catastrophic
failure. The applications in this field relate not only to missiles in warfare,
but also to their safe ground transportation and storage whilst on land or ship
and to manned space flight. ‘It will be interesting to see whether a model material
can be devised that replicates the microstructural features (and is therefore
safe to study in our Scanning Electron Microscope). When combined with our fracture
models, this may enable us to control and manipulate the propellant’s microstructure,
leading to a stable and safer fuel material,’ comments Dr Beaumont.